Such boring machines comprise a drill rod with a rotary and/or percussion or vibratory drive and with a very wide variety of drill heads. The drill rod is usually mounted on a carriage guided on rails and connected to a linear drive and has a rotary or rotary/percussion drive by means of which the rod can be caused to rotate and optionally also be forced into the ground.
There are various methods and machines for directional boring in soft ground, in most of which a rotatably mounted drill rod carries a drill head having a steering face inclined to the axis of the rod, According to EP-A 0 195 559 such drill heads may also be provided with cutting jet nozzles. When the rod is rotating, i.e. when boring straight ahead, both the steering head and the cutting jets dig into the ground, insofar as it is sufficiently workable. With hard ground, and particularly in rock formations, this is not the case, since the cutting jets then lose their digging power and the steering face remains ineffective.
In order to initiate a curve boring with the known device the rotation of the drill head is interrupted and the drill head, still subjected to a forward driving force, is deflected, depending on the inclination of the steering face relative to the axis of the rod, to the side opposite the steering face. As soon as the desired curved path has been traversed, the drill head is again caused to rotate, thereby neutralising the deflecting action of the inclined face. This is also done for a short time if a correction of the curved path is found to be necessary.
Moreover, cutting-jet drilling also suffers from the disadvantage that as a result of the high fluid pressure washouts and consequent cave-ins may occur.
It is generally necessary to adjust the angular position of the drill head repeatedly in this way in the course of a (fairly long) curve. Hence the rod, which is otherwise not rotating as it follows a curve, always has to take a plurality of angled steps. This results in a zigzag or corkscrew-shaped borehole in the ground, but no precise curved path.
The angular position of the inclined face of the drill head at any time depends on the direction of curvature of the borehole to be produced, but the inclined face is always on the inside of the curved path, where it acts in effect like a pivot, while as the rod or the drill head is driven forward mechanically by pushing and/or striking but without rotation the opposite side to the inclined face acts in the ground as a shoulder or guide shoe sliding along a guide plank. During the rotation-free curve boring the soil in front of the drill head is forced aside by the drill head and/or is excavated to a greater or less extent by means of a sharp jet of fluid. However, this is only possible in the case of soils that are not too hard and are free from obstructions, and can also be forced aside.
This procedure, however, fails in rock formations, since during the curve boring the drill head does not rotate and is advanced only by pushing. Hard soils and rock formations therefore require boring machines having digging or cutting tools which are driven independently of any rotation of the drilling head and which permit mechanical excavation of the ground during the curve boring, i.e. when the drill head is not rotating.
A machine of this kind is known from U.S. Pat. No. 3,878,903. This machine has a single-strand rod having a mud motor driving the digging tool. This motor is driven by a fluid supplied through the hollow rod, which normally passes out of the drill head into the ground and is then lost.
Boring systems are also known in which the driving fluid is returned and re-used. In each case, however, having regard to the high speed of rotation and a correspondingly high torque at the digging tool, large amounts of fluid are required, which must be made available, pumped round, returned and recycled or disposed of.
A further disadvantage of these machines is that because of lack of space in the drill head the size of the motor and consequently the driving power is limited. Moreover in case of damage to the mud motor the whole drill rod must be withdrawn in order to remove or repair the motor. In the case of boreholes 200 m or more in length this takes a great deal of time and leads to correspondingly long interruptions in the operation.
EP-A 0 247 767 also describes a drill head connected to a rotary/thrust rod and having an inclined face which permits straight-ahead boring so long as the drill head rotates uniformly, and in the absence of rotation curve boring by lateral displacement of the soil located in front of the drill head.
The process concerned here, however, is purely one of displacement, which is quite unsuitable for hard and rocky ground formations and in addition, because of the lateral displacement of the ground surrounding the borehole, can lead to damage to other supply lines or at the surface, for example to road surfaces. Accordingly, when boring with such machines it is always necessary to maintain an adequate margin of safety.
The known steerable boring machines having a digging tool driven via an inner rod share the constructional feature that the tool is in the form of a face tool and has an axis of rotation which is either parallel to or is inclined at a more or less acute angle to the axis of rotation of the rod. The radius of curvature when curve boring is therefore fixed once for all: it is always determined by the eccentricity or the inclination of the axis of rotation of the tool relative to the axis of rotation of the rod. Curve boring with a particular radius of curvature therefore requires a correction of the respective angular setting of the eccentricity and therefore necessarily results in a zigzag or corkscrew-shaped borehole, since in describing a curve numerous corrective changes in the angular setting of the eccentricity are needed.
This has an extremely adverse effect when subsequently widening the pilot bore by means of an enlarging head and/or drawing a pipeline into the pilot bore, since precisely in the particularly critical curve region the borehole follows an irregular course and has nonuniform walls, which present a high frictional resistance to the enlarging head and/or to the pipe which is being drawn in. This requires increased technical outlay when enlarging and drawing-in. Added to this is the instability of such borehole walls, which is associated with the risk of cave-ins, which increase the difficulties in widening the borehole and drawing in pipes, and in particular gives rise to the risk of damage to the pipe as it is drawn in.
A further European patent application, EP-A 0 674 093, describes a steerable boring machine with an eccentrically disposed drill head. This drill head is driven via an inner rod disposed in an outer pipe. The outer pipe is connected to a rotary drive and has a sliding block or shoe fitted on to it at its front end which forces the drill head to one side, i.e. into a curved course, in the event of interruption of the rotation of the outer pipe. However, associated with the sliding block comes the disadvantage of non-uniformity in the walls of the borehole, which can lead to difficulties in the introduction of a product pipe into the borehole and also to caving in of the soil.
Generally speaking, in the known boring machines the driven tool is primarily designed for curve boring: substantially exact straight-ahead boring is therefore in most cases not possible, so that also in the case of straight-ahead boring directional corrections are needed from time to time. Consequently in straight-ahead boring a more or less pronounced zigzag course of the borehole results, which can lead to great difficulties when introducing a product pipe, for example as a follower pipe.